1. Field of the Invention
The present invention relates to an energy absorber and, in particular, to an energy absorber that absorbs tensile energy and deploys irreversibly at or close to a constant force.
2. Background Art
Tension energy shock absorbers are often used to assist the absorption of energy in constrained or partially constrained lines. For example, fall arrest applications require energy from a falling body to be absorbed by a line such as a rope or wire which is usually attached to a strong structure at one end or both ends. In such situations, it is desirable to achieve a low combination of stretch in the line and line tension. Lowering line stretch reduces the distance the body falls before arrest, and also reduces the fall energy. Lowering line tension reduces the loading on the line and also on the anchor or anchors constraining the line. Another example of energy absorption in constrained lines is vehicle crash barriers that absorb vehicle kinetic energy. Lower line stretch reduces the degree to which a vehicle can move across a crash barrier. Lower line loads reduce the likelihood of line and/or anchorage failure in the event of a crash.
The amount of energy absorption in constrained or partially constrained lines is determined by the product of stretch in the line and the line tension. Typically, line stretch is elastic such that the amount of stretch increases in proportion to the tension in the line. The energy absorbed is therefore the average line tension or half the maximum line tension multiplied by the stretch. However, in order to minimise the combination of line stretch and line tension, the ideal line system would absorb energy by stretching at a predetermined line tension where the energy absorbed is the predetermined line tension multiplied by the line stretch. This would absorb the same energy as the elastic line system for a given maximum line tension but require only half the line stretch. Also, such a system would be able to limit maximum line tension to the predetermined force at which stretch occurs.
In practice, it is difficult to achieve this ideal, but significant improvement can be made in energy absorption efficiency with respect to the combination of line stretch and line tension by combining low stretch line with an energy absorber that deploys by stretching at a predetermined force. If the extent of deployment stretch in such an absorber is sufficiently large, it could also effectively limit line tension to the predetermined deployment force for all foreseeable energy absorption situations. This is important for establishing with a high degree of certainty safe design criteria for line systems and anchors.
In energy absorbers for use in fall arrest systems it is a normal requirement that the energy absorber be able to support double the peak deployment force after full deployment.
In conventional energy absorbers that deploy at a constant force, the component which is deployed is typically straight, being housed within a further straight component prior to deployment. The overall length of such an absorber prior to deployment is therefore greater than the extent of deployment. Such energy absorbers typically consist of a component, preferably having a spherical or part-spherical leading portion, which is pulled through a length of tube having a bore smaller in diameter than the outer dimension of the leading portion of said component, such that a force is required effectively to extrude the bore of the tube. One such energy absorber is described in the present Applicants granted European Patent No. EP 0 605 538.
In view of the foregoing analysis, it will be clear to persons skilled in the art that, in applications in which large deployment extents are required, the overall length of the energy absorber also needs to be large.
This is undesirable, not only from the point of view of cost, but also because in many applications such as fall arrest it is important to gain access to a constrained or partially constrained line close to the constraining anchors. Typically, large deployment extents are useful for containing line loads and also ensuring that line loads never exceed the predetermined deployment force of the energy absorber for all foreseeable situations, and provide a useful energy absorption surplus as a contingency against unforeseeable circumstances.